Individual Actuation within a Source Subarray

ABSTRACT

Source element of a source subarray can be individually actuated according to an actuation sequence. The actuation sequence can be at least partially based on a relative position of each of the source elements within a particular geometry of the source subarray with respect to a previously actuated source element and a towing velocity of the source subarray.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.15/344,669 filed Nov. 7, 2016, published as U.S. Publication No.2017/0176620 on Jun. 22, 2017, which will issue as U.S. Pat. No.10,495,770 on Dec. 3, 2019, which claims priority to U.S. ProvisionalApplication 62/268,020, filed Dec. 16, 2015, which is incorporated byreference.

BACKGROUND

In the past few decades, the petroleum industry has invested heavily inthe development of marine survey techniques that yield knowledge ofsubterranean formations beneath a body of water in order to find andextract valuable mineral resources, such as oil. High-resolution imagesof a subterranean formation are helpful for quantitative interpretationand improved reservoir monitoring. For a typical marine survey, a marinesurvey vessel tows one or more sources below the water surface and overa subterranean formation to be surveyed for mineral deposits. Receiversmay be located on or near the seafloor, on one or more streamers towedby the marine survey vessel, or on one or more streamers towed byanother vessel. The marine survey vessel typically contains marinesurvey equipment, such as navigation control, source control, receivercontrol, and recording equipment. The source control may cause the oneor more sources, which can be air guns, marine vibrators,electromagnetic sources, etc., to produce signals at selected times.Each signal is essentially a wave called a wavefield that travels downthrough the water and into the subterranean formation. At each interfacebetween different types of rock, a portion of the wavefield may berefracted, and another portion may be reflected, which may include somescattering, back toward the body of water to propagate toward the watersurface. The receivers thereby measure a wavefield that was initiated bythe actuation of the source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an elevation or xz-plane view of marine surveying inwhich acoustic signals are emitted by a source for recording byreceivers for processing and analysis in order to help characterize thestructures and distributions of features and materials underlying thesolid surface of the earth.

FIG. 2 illustrates a top view of marine surveying.

FIG. 3 illustrates a perspective view of a source element duringactuation while the source element travels in a direction at vesselspeed.

FIG. 4 illustrates a plot of a near-field signature of a source elementmeasured by a pressure sensor located in close proximity to the sourceelement.

FIG. 5 illustrates a perspective view of a source element a short timeafter the actuation depicted in FIG. 3.

FIG. 6 illustrates a plot of a source wavefield in a vertical direction.

FIG. 7A illustrates an actuated source element as depicted in the FIG. 3surrounded by three other source elements of a source subarray travelingin the same direction.

FIG. 7B illustrates source elements passing through a foam of small airbubbles.

FIG. 8A illustrates an xz-plane view of a source subarray with sourceelements arranged in a substantially elliptical shape.

FIG. 8B illustrates a yz-plane view of a source subarray with sourceelements arranged in a substantially elliptical shape and an exampleactuation sequence.

FIG. 9A illustrates an xz-plane view of a source subarray with sourceelements arranged in a substantially rectangular shape.

FIG. 9B illustrates a yz-plane view of a source subarray with sourceelements arranged in a substantially rectangular shape and an exampleactuation sequence.

FIG. 10A illustrates an xz-plane view of a source subarray with sourceelements arranged in a “V” shape.

FIG. 10B illustrates a yz-plane view of a source subarray with sourceelements arranged in a “V” shape and an example actuation sequence.

FIG. 11A illustrates an xz-plane view of a source subarray with sourceelements arranged along three lines.

FIG. 11B illustrates a yz-plane view of a source subarray with sourceelements arranged along three lines and an example actuation sequence.

FIG. 12A illustrates an xz-plane view of a source subarray with sourceelements arranged along a single line.

FIG. 12B illustrates a yz-plane view of a source subarray with sourceelements arranged along a single line and an example actuation sequence.

FIG. 13 illustrates a yz-plane view of a source array comprising asource subarray and additional source subarrays.

FIG. 14 illustrates a method for individual actuation within a sourcesubarray.

FIG. 15 illustrates a system for individual actuation within a sourcesubarray.

DETAILED DESCRIPTION

This disclosure is related generally to the field of marine surveying.Marine surveying can include, for example, seismic and/orelectromagnetic surveying, among others. For example, this disclosuremay have applications in marine surveying, in which one or more sourceelements are used to generate wave-fields, and sensors (towed and/orocean bottom) receive energy generated by the source elements andaffected by the interaction with a subsurface formation. The sensorsthereby collect survey data, which can be useful in the discovery and/orextraction of hydrocarbons from subsurface formations.

The general term source is used herein to include source elements,source units, source arrays, and source subarrays. Source elements canbe individual sources such as an air gun, electromagnetic source, ormarine vibrator, among others. Source units can be multiple sourceelements that are actuated together. Source arrays can be multiplesource elements and/or multiple source units that can be actuatedseparately. A source array can also comprise an array of source elementspartitioned into subsets of source elements called source subarrays. Asource subarray is a portion of a source array such as those sourceelements that are disposed along a cable towed by a vessel. The sourcesubarrays can be towed approximately parallel to the direction that thevessel is traveling. Source elements can have different sizes andmultiple source elements may be disposed in a same position along thecable. For example, one or more source elements can be arranged suchthat they are coupled to the source subarray at a same position.

Source subarrays can have different lengths, such as ten to twentymeters in length. Source elements of a source unit can be coupled at asame position so that they can be actuated simultaneously and/orconcurrently as the source unit, which may also be referred to as acluster. The source elements that are coupled to the source subarray ata same position can be actuated independently of other source elementsthat are coupled to the same position. For example, one or more sourceelements that are coupled to a same position along the source subarraycan be actuated in the event that one of the other source elements thatare coupled to the same source location along the cable fails toactuate.

In some approaches to marine geophysical data acquisition, sourceelements towed along a source subarray may be actuated simultaneously.If the source elements are actuated at different times, then the watercolumn surrounding one or more of the source elements may be affected bya disturbance, such as air bubbles, caused by another source element.For example, the water column surrounding one or more of the sourceelements may be affected by air in the water as a result of theactuation of a source element at an earlier time as discussed below withreference to FIGS. 3-7B. The effects of air bubbles caused by therelease of air by the source elements may cause complex and/orunpredictable effects on a wavefield emitted by a source element.

Source elements can be actuated at different times in a continuous ornear-continuous sequence so that an actuation sequence can cycle or“loop” through source elements that are available to be actuated. Anactuation sequence can include individually actuating each source of asource subarray at different times but with as close as a separation intime as possible. Individually actuating each source of a sourcesubarray at different times but with as close as a separation in time aspossible can cause a source wavefield to approach white noise, which canstabilize the deconvolution of the source wavefield. An actuationsequence can include avoiding actuating a source element in a locationthat might be contaminated with a disturbance from a previous actuationof a source element.

As used herein, “near-continuous” can include without meaningful breaksin the actuation sequence or between the actuations of individual sourceelements. As would be understood by one of ordinary skill in the artwith the benefit of this disclosure, operational circumstances can causeintermittent gaps between actuations (due to equipment failure, etc.),and “near-continuous actuation sequence” and “near-continuous actuationof the source elements” should be read to include actuations withintermittent or periodic gaps, whether planned or unplanned as well asactuations without intermittent or periodic gaps, thus including“continuous actuation sequence” and “continuous actuation of the sourceelements.” For simplicity, the term “near-continuous” and“near-continuously” will be used herein and do not exclude “continuous”or “continuously.”

A source element may be actuated in a location where another sourceelement was previously actuated because the source element is movingthrough the water. Because a source array may be towed behind the vesselwith some speed, it may be desirable to have a gap in time betweenactuations of the source elements that is long enough to avoid actuatinga source element in a location that is close to the location of aprevious actuation of a source element. For example, if the towingvelocity of a source subarray is two meters per second (m/s), thedistance between a first source element and a second source element ofthe source subarray is two meters, and the second source element isactuated one second after the actuation of the first source element,then the second source element would be actuated in the location wherethe first source element was actuated. The water in the area surroundingthe second source element may be contaminated with a disturbance. Toactuate the second source element away from the disturbance, for exampleat a distance of at least two meters away from the location where thefirst source element was actuated, the second source element may beactuated at least two seconds after than the actuation of the firstsource element. However, this time difference may be such that theactuations of the source elements are no longer considered to be acontinuous or near-continuous actuation sequence.

In an effort to overcome the above described shortcomings, at least oneembodiment in accordance with the present disclosure can includearranging the source elements associated with a particular sourcesubarray in a particular geometry and actuating each of the sourceelements according to an actuation sequence. The actuation sequence canbe at least partially based on a relative position of each sourceelement within the particular geometry of the source subarray withrespect to a previously actuated source element, and a towing velocityof the source subarray. Thus, a marine survey can be conducted viacontinuous or near-continuous actuations of the source elements and thetime interval between actuations of individual source elements can bereduced or minimized, for example one second or less.

It is to be understood the present disclosure is not limited toparticular devices or methods, which may, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. As used herein, the singular forms “a”, “an”, and “the”include singular and plural referents unless the content clearlydictates otherwise. Furthermore, the word “may” is used throughout thisapplication in a permissive sense (having the potential to, being ableto), not in a mandatory sense (must). The term “include,” andderivations thereof, mean “including, but not limited to.” The term“coupled” means directly or indirectly connected.

The figures herein follow a numbering convention in which the firstdigit or digits correspond to the drawing figure number and theremaining digits identify an element or component in the drawing.Similar elements or components between different figures may beidentified by the use of similar digits. As will be appreciated,elements shown in the various embodiments herein can be added,exchanged, and/or eliminated so as to provide a number of additionalembodiments of the present disclosure. In addition, as will beappreciated, the proportion and the relative scale of the elementsprovided in the figures are intended to illustrate certain embodimentsof the present invention, and should not be taken in a limiting sense.

FIG. 1 illustrates an elevation or xz-plane 130 view of marine surveyingin which acoustic signals are emitted by a source 126 for recording byreceivers 122 for processing and analysis in order to help characterizethe structures and distributions of features and materials underlyingthe solid surface of the earth. FIG. 1 shows a domain volume 102 of theearth's surface comprising a solid volume 106 of sediment and rock belowthe solid surface 104 of the earth that, in turn, underlies a fluidvolume 108 of water having a water surface 109 such as in an ocean, aninlet or bay, or a large freshwater lake. The domain volume 102 shown inFIG. 1 represents an example experimental domain for a class of marinesurveys, such as marine seismic surveys. FIG. 1 illustrates a firstsediment layer 110, an uplifted rock layer 112, second, underlying rocklayer 114, and hydrocarbon-saturated layer 116. One or more elements ofthe solid volume 106, such as the first sediment layer 110 and the firstuplifted rock layer 112, can be an overburden for thehydrocarbon-saturated layer 116. In some instances, the overburden mayinclude salt.

FIG. 1 shows an example of a marine survey vessel 118 equipped to carryout marine surveys. In particular, the marine survey vessel 118 can towone or more streamers 120 (shown as one streamer for ease ofillustration) generally located below the water surface 109. Thestreamers 120 can be long cables containing power and data-transmissionlines (electrical, optical fiber, etc.) to which receivers may beconnected. In one type of marine survey, each receiver, such as thereceiver 122 represented by the shaded disk in FIG. 1, comprises a pairof sensors including a motion sensor that detects particle displacementwithin the water by detecting particle motion variation, such asvelocities or accelerations, and/or a hydrophone that detects variationsin pressure. The streamers 120 and the marine survey vessel 118 caninclude sophisticated sensing electronics and data-processing facilitiesthat allow receiver readings to be correlated with absolute positions onthe water surface and absolute three-dimensional positions with respectto a three-dimensional coordinate system. In FIG. 1, the receivers alongthe streamers are shown to lie below the water surface 109, with thereceiver positions correlated with overlying surface positions, such asa surface position 124 correlated with the position of receiver 122. Themarine survey vessel 118 can also tow one or more sources 126 thatproduce acoustic signals as the marine survey vessel 118 and streamers120 move across the water surface 109. Sources 126 and/or streamers 120may also be towed by other vessels, or may be otherwise disposed influid volume 108. For example, receivers may be located on ocean bottomcables or nodes fixed at or near the solid surface 104, and sources 126may also be disposed in a nearly-fixed or fixed configuration. For thesake of efficiency, illustrations and descriptions herein show seismicreceivers located on streamers, but it should be understood thatreferences to seismic receivers located on a “streamer” or “cable”should be read to refer equally to seismic receivers located on a towedstreamer, an ocean bottom receiver cable, and/or an array of nodes.

FIG. 1 shows an expanding, spherical acoustic signal, illustrated assemicircles of increasing radius centered at the source 126,representing a down-going wavefield 128, following an acoustic signalemitted by the source 126. The down-going wavefield 128 is, in effect,shown in a vertical plane cross section in FIG. 1. The outward anddownward expanding down-going wavefield 128 may eventually reach thesolid surface 104, at which point the outward and downward expandingdown-going wavefield 128 may partially scatter, may partially reflectback toward the streamers 120, and may partially refract downward intothe solid volume 106, becoming elastic acoustic signals within the solidvolume 106.

FIG. 2 illustrates a top view of marine surveying. FIG. 2 shows anexample of a marine survey vessel 218, analogous to the marine surveyvessel 118 illustrated in FIG. 1, equipped to carry out marine surveys.The marine survey vessel 218 can tow one or more streamers 220,analogous to the streamer 120 illustrated in FIG. 1. The streamers caninclude one or more receivers 222, analogous to the receivers 122illustrated in FIG. 1. The marine survey vessel can tow one or moresources 226, analogous to the sources 126 illustrated in FIG. 1. Therecorded data can be three-dimensional in that it includes data fromwavefields traveling in both an inline direction 229 and a cross-linedirection 231, plus depth. The inline direction 229 is generally in linewith the one or more sources 226 with respect to a direction of travelof the marine survey vessel 218 and/or with respect to a length ofreceivers 222 along a streamer 220 or ocean bottom cable. The cross-linedirection 231 is generally perpendicular to the inline direction 229 andcrosses the length of receivers 222 along a streamer 220 or ocean bottomcable. The streamers 220 or ocean bottom cables are generally spacedapart in the cross-line direction 231. In at least one embodiment, thestreamers 220 can be towed in a curved path.

The marine survey vessel 218 can include a control system and arecording system, which may be separate systems that communicate databetween each other, or they may be sub-systems of an integrated system.The control system can be configured to selectively actuate the sources226, while the recording system can be configured to record the signalsgenerated by receivers 222 in response to the energy imparted into thewater and thereby into subterranean material formations below the solidsurface. The recording system can be configured to determine and recordthe geodetic positions of the sources and the receivers 222 at any time.

Source actuation and signal recording by the receivers 222 may berepeated a plurality of times while the marine survey vessel 218 movesthrough the water. Each actuation record may include, for each receiver222, signals corresponding to the energy produced by the source 226.

FIG. 3 illustrates a perspective view of a source element 351 duringactuation while the source element 351 travels in a direction 352 at atowing velocity. As the source element 351 is actuated, air is rapidlyforced out through one or more openings located on an end, or along theside, of the source element 351 forming a complex combination of largebubbles, such as the large bubble 356, and many smaller bubbles 354forming a foam of air, shown as foam 350, around the larger bubbles.High pressure in the large bubble 356 generates acoustic pressure wavesthat radiate outward. In other words, when the large bubble 356 isinjected into the water from the source element 351 there is a radialdisplacement of water from the center of the large bubble 356 and apressure disturbance propagates outward. As the large bubble 356 expandsthe pressure of the air in the large bubble 356 drops below that of thesurrounding fluid, but inertia causes the large bubble 356 to overexpand so that the air pressure in the large bubble 356 is less than thepressure of the surrounding water. Then the large bubble 356 contractsdue to the pressure of the surrounding water. This process of expansionand contraction continues with the large bubble 356 oscillating throughmany cycles with pressure waves radiating outward into the water. Theamplitude of the pressure wave decreases with time.

FIG. 4 illustrates a plot of a near-field signature 464 of a sourceelement 351 measured by a pressure sensor located in close proximity tothe source element 351. The horizontal axis 462 represents time, thevertical axis 460 represents pressure, and the curve 464 represents thenear-field signature of the pressure wave emitted from the sourceelement 351. The near-field signature 464 represents changes in thepressure amplitude of the bubble output from the source element 351. Thefirst peak 466 corresponds to the initial build-up and release ofpressure in a bubble output from the source element 351 into the water,after which, subsequent peaks represent a decrease in amplitude withincreasing time. The near-field signature reveals that the pressurefalls below the hydrostatic pressure, p_(h), between peaks. The bubbleoscillation amplitude decreases as time passes and the bubble period ofoscillation is not constant from one cycle to the next. In other words,the bubble motion is not simple harmonic motion. The chamber volume ofsource element 351 determines the associated near-field signature, whichis also influenced by the pressure waves created by other sourceelements 351 of the source subarray. In general, the larger the chambervolume the larger the peak amplitudes and the longer the bubble periodsof the associated near-field signatures.

FIG. 5 illustrates a perspective view of a source element 551 a shorttime after the actuation depicted in FIG. 3. As the source element 551continues to move in the direction 552, the large bubble 556 risesthrough the water faster than the smaller bubbles 554. The water createsdrag that essentially stops the large bubble 556 and the smaller bubbles554 from moving forward behind the source element 551. Over time thefoam 550 expands to fill an air/water volume with many of the smallerbubbles 554 remaining in the air/water volume around and above thelocation where the source element 551 was actuated.

The pressure waves output from the source elements 551 combine to form asource wavefield, which is the acoustic signal that illuminates asubterranean formation as described above with reference to FIG. 1. Thesource elements 551 within a source subarray may be selected withdifferent chamber volumes, spacings, and positions in order to generatea desirable source wavefield.

FIG. 6 illustrates a plot of a source wavefield in a vertical direction.The horizontal axis 662 represents time, the vertical axis 660represents pressure, and the curve 664 represents a resulting far-fieldamplitude of the source wavefield, for the case in which all the sourceelements 551 in the array are fired simultaneously. The far-fieldamplitude 664 has a large primary peak 672 and a ghost peak 674 followedin time by very small amplitude oscillations 676. The primary peak 672represents the portion of the source wavefield that travels directly tothe subterranean formation while the ghost peak 674 represents theportion of the source wavefield that is reflected from the water surfaceand is responsible for source ghost contamination of the wavefieldsmeasured by the receivers 122, as illustrated in FIG. 1.

A source wavefield created by simultaneously activating the sourceelements of a moving source array are not adversely affected by airbubbles created by previous simultaneous actuations of the source arraybecause the air-bubbles from previous actuations remain at the locationwhere the source array was previously actuated. On the other hand, whenthe source elements of a moving source array are actuated at differenttimes within a short time interval (a few seconds or less), the watercolumn surrounding a next to be actuated source element may be filledwith air bubbles caused by one or more neighboring source elements thatwere actuated at earlier times. The air-bubbles may create very complexand unpredictable effects on the wavefield emitted by the source elementto be actuated next. Some of these effects may be related to thecomplexity of the medium caused by the mixture of air bubbles and water,with air bubbles of different sizes and large density and velocitycontrasts between air and water, causing scattering, attenuation andpropagation effects. As a result, this component of the source wavefieldmay become unpredictable, variable, and/or chaotic.

FIG. 7A illustrates an actuated source element 751 as depicted in theFIG. 3 surrounded by three other source elements 751 of an arraytraveling in the same direction 752. FIG. 7B illustrates source elements751 passing through a foam 750 of small air bubbles. The large bubble756 and the small air bubbles 754 create an air/water volume in closeproximity to the source elements 751. The air/water volume impactsbubble oscillation of air injected by the source elements 751. As aresult, acoustic energy that travels downward and away from the sourceelements 751 passes through the air/water volume and is subjected tounpredictable perturbations. In addition, the source ghost (watersurface reflected energy) created by the source elements 751 cannot beaccurately estimated because the air/water volume above the sourceelements 751 creates unpredictable perturbations in the acoustic energytraveling upward from the source elements 751. The effects may make itdifficult to accurately determine the total three-dimensional wavefieldemitted from a source with the source elements actuated at differenttimes, which in turn leads to an inevitable reduction in the quality inany final seismic images.

Methods and systems described below are directed to particulargeometries of source elements of a source subarray and actuationsequences. An example of a source element is an air gun. A sourcesubarray can be composed of a single source element, two sourceelements, or more. An actuation sequence can be at least partially basedon a relative position of each source element within a particulargeometry of a source subarray with respect to a previously actuatedsource element. An actuation sequence can also be at least partiallybased on the towing velocity of the source subarray.

Examples of the particular geometry include, but are not limited to,those illustrated in FIGS. 8A-12B. For example, the particular geometrycan comprise four of the source elements in a single inline positionalong the source subarray. Another example of the particular geometrycan comprise a first source element at a first cross-line position and asecond source element at a second cross-line position, wherein the firstcross-line position is different than the second cross-line position.The particular geometry can also comprise a first source element at afirst depth and a second source element at a second depth, wherein thefirst depth is different than the second depth.

FIG. 8A illustrates an xz-plane 830 view of a source subarray 880 withsource elements 851 arranged in a substantially elliptical shape. Asused herein, “arranged in a substantially elliptical shape” is intendedto mean arranged along a curve that surrounds two focal points such thatthe sum of the distances to the two focal points is substantiallyconstant for every point on the curve. The source subarray 880 can becoupled to a floatation device 882, which can be a buoy.

FIG. 8B illustrates a yz-plane 886 view of a source subarray 880 withsource elements 851 arranged in a substantially elliptical shape and anexample actuation sequence. The embodiment shown in FIG. 8B comprisestwelve source elements 851 equally spaced along a circle. A circle is asubstantially elliptical shape where the two focal points are in thesubstantially same location. However, embodiments in accordance with thepresent disclosure are not so limited and can include the sourceelements 851 having varied spacing and can be along any ellipticalshape. Additionally, the quantity of source elements 851 comprising thesource subarray 880 is not limited to twelve.

The numbers within the circles representing the source elements 851correspond to an example actuation sequence. As used herein, “actuationsequence” is intended to mean a sequence, or an order, of actuations ofsource elements, such as the source elements 851, of a source subarray,such as the source subarray 880. Each of the source elements 851 can beactuated individually such that the source element 851 labeled “1” isactuated first, then the source element 851 labeled “2” is actuatedsecond, and so on until all of the source elements 851 have beenactuated. The actuation of the source elements 851 can be repeatedaccording to the actuation sequence after all of the source elements 851of the source subarray 880 have been actuated.

FIG. 9A illustrates an xz-plane 930 view of a source subarray 981 withsource elements 951 arranged in a substantially rectangular shape. Asused herein, “arranged in a substantially rectangular shape” is intendedto mean arranged along a polygon with four sides where the four sidesform four angles of substantially ninety degrees. The source subarray981 can be coupled to a floatation device 982, which can be a buoy.

FIG. 9B illustrates a yz-plane 986 view of a source subarray 981 withsource elements 951 arranged in a substantially rectangular shape and anexample actuation sequence. The embodiment shown in FIG. 9B comprisestwelve source elements 951 equally spaced along a square. A square is asubstantially rectangular shape where the four sides have substantiallythe same length. However, embodiments in accordance with the presentdisclosure are not so limited and can include the source elements 951having varied spacing and can be along any rectangular shape.Additionally, the quantity of source elements 951 comprising the sourcesubarray 981 is not limited to twelve.

The numbers within the circles representing the source elements 951correspond to an example actuation sequence. Each of the source elements951 can be actuated individually such that the source element 951labeled “1” is actuated first, then the source element 951 labeled “2”is actuated second, and so on until all of the source elements 951 havebeen actuated. The actuation of the source elements 951 can be repeatedaccording to the actuation sequence after all of the source elements 951of the source subarray 981 have been actuated.

FIG. 10A illustrates an xz-plane 1030 view of a source subarray 1083with source elements 1051 arranged in a “V” shape. As used herein,“arranged in a ‘V’ shape” is intended to mean arranged along two lineswhere the two lines share a common endpoint and the angle between thetwo lines is less than one hundred eighty degrees. The source subarray1083 can be coupled to one or more floatation devices 1082, which can bebuoys.

FIG. 10B illustrates a yz-plane 1086 view of a source subarray 1083 withsource elements 1051 arranged in a “V” shape and an example actuationsequence. The embodiment shown in FIG. 10B comprises twelve sourceelements 1051 equally spaced along the two lines. Embodiments inaccordance with the present disclosure can include the source elements1051 having varied spacing along the two lines. Additionally, thequantity of source elements 1051 comprising the source subarray 1083 isnot limited to twelve.

The numbers within the circles representing the source elements 1051correspond to an example actuation sequence. Each of the source elements1051 can be actuated individually such that the source element 1051labeled “1” is actuated first, then the source element 1051 labeled “2”is actuated second, and so on until all of the source elements 1051 havebeen actuated. The actuation of the source elements 1051 can be repeatedaccording to the actuation sequence after all of the source elements1051 of the source subarray 1083 have been actuated.

FIG. 11A illustrates an xz-plane 1130 view of a source subarray 1184with source elements 1151 arranged along three lines. The sourcesubarray 1184 can be coupled to a floatation device 1182, which can be abuoy.

FIG. 11B illustrates a yz-plane 1186 view of a source subarray 1184 withsource elements 1151 arranged along three lines and an example actuationsequence. The lines of any source subarray in accordance with thepresent disclosure, such as the source subarray 1184, can besubstantially vertical with respect to a water surface and can besubstantially parallel to each other. The embodiment shown in FIG. 11Bcomprises twelve source elements 1151 equally divided amongst threelines and equally spaced along the three lines. The cross-lineseparation between a first (left) line and a second (center) line can beequal to the cross-line separation between the second (center) line anda third (right) line. However, embodiments in accordance with thepresent disclosure are not so limited. The particular geometry cancomprise the source elements arranged along at least one line. Thesource elements 1151 can have varied spacing along the at least oneline. Additionally, the quantity of source elements 1151 comprising thesource subarray 1184 is not limited to twelve.

The numbers within the circles representing the source elements 1151correspond to an example actuation sequence. Each of the source elements1151 can be actuated individually such that the source element 1151labeled “1” is actuated first, then the source element 1151 labeled “2”is actuated second, and so on until all of the source elements 1151 havebeen actuated. The actuation of the source elements 1151 can be repeatedaccording to the actuation sequence after all of the source elements1151 of the source subarray 1184 have been actuated.

FIG. 12A illustrates an xz-plane 1230 view of a source subarray 1285with source elements 1251 arranged along a single line. The sourcesubarray 1285 can be coupled to a floatation device 1282, which can be abuoy.

FIG. 12B illustrates a yz-plane 1286 view of a source subarray 1285 withsource elements 1251 arranged along a single line and an exampleactuation sequence. The embodiment shown in FIG. 12B comprises twelvesource elements 1251 equally spaced along a single vertical line.However, embodiments in accordance with the present disclosure are notso limited and can include the source elements 1251 having variedspacing and can be along a single line at an angle of less than ninetydegrees with respect to a water surface. Additionally, the quantity ofsource elements 1251 comprising the source subarray 1285 is not limitedto twelve.

The numbers within the circles representing the source elements 1251correspond to an example actuation sequence such that the actuationsbegin with an initial source element 1251 that is disposed at an inlineposition along the single line that is closest to the water surfacefollowed by actuating, in order along the single line, the sourceelements positionally subsequent to the initial source element. Each ofthe source elements 1251 can be actuated individually such that thesource element 1251 labeled “1” is actuated first, then the sourceelement 1251 labeled “2” is actuated second, and so on until all of thesource elements 1251 have been actuated. The actuation of the sourceelements 1251 can be repeated according to the actuation sequence afterall of the source elements 1251 of the source subarray 1285 have beenactuated.

FIG. 13 illustrates a yz-plane 1330 view of a source array 1340comprising a source subarray 1380-1 and additional source subarrays1380-2 and 1380-3. The source array 1340 can comprise a source subarray1380-1 and an additional source subarray 1380-2. The source array 1340can comprise more than one additional source subarray 1380-2 and 1380-3as depicted in FIG. 13. The source subarrays 1380-1, 1380-2, and 1380-3can be analogous to the source subarray 880 as depicted in FIGS. 8A and8B; however any source subarray in accordance with the presentdisclosure can be used. Although the source array 1330 can comprise thesource subarray 1380-1 and the additional source subarrays 1380-2 and1380-3 can each have a different particular geometry, such as theparticular geometries of the source subarrays 880 and 981, it can bebeneficial to use a single particular geometry in the source array 1330.

The source elements 1351 of the source array 1340 can be actuatedaccording to the actuation sequence for the source subarray 1380-1 andthe additional source subarrays 1380-2 and 1380-3 such that all of thesource elements 1351 of the source subarray 1380-1 are actuated beforeany source element 1351 of the additional source subarrays 1380-2 and1380-3 is actuated. The source elements 1351 of the additional sourcesubarray 1380-2 can be then be actuated according to the actuationsequence.

As illustrated in FIG. 13, an actuation sequence for the source array1340 can include actuating a first source element 1351 of a sourcesubarray 1380-1 (denoted as 1A) and a first source element 1351 of eachadditional source subarray 1380-2 and 1380-3 (denoted as 1B and 1C,respectively) before actuating a second source element 1351 of thesource subarray 1380-1 (denoted as 2A) and a second source element 1351(denoted as 2B and 2C, respectively) of each additional source subarray1380-2 and 1380-3. In other words, the actuation sequence can beactuating the source elements 1351 in the sequence of: 1A, 1B, 1C, 2A,2B, 2C, and so on. After all of the source elements 1351 of the sourcearray 1340 are actuated, the actuation sequence can then be repeated.Such an actuation sequence can minimize the time between the actuationsof the source elements 1351. As an example, if each source element 1351of the source array 1340 is actuated with a mean time interval of 0.1seconds between actuations, then the mean time interval between theactuations of the source elements 1351 of the source subarray 1380-1will be 0.3 seconds (0.1 seconds between the actuations of sourceelements 1351 denoted as 1A and 1B, 0.1 seconds between the actuationsof source elements 1351 denoted as 1B and 1C, and 0.1 seconds betweenthe actuations of source elements 1351 denoted as 1C and 2A, so thatthere are 0.3 seconds between the actuations of source elements 1351denoted as 1A and 2A).

FIG. 14 illustrates a method for individual actuation within a sourcesubarray. The method can comprise, at block 1488, individually actuatingsource elements of a source subarray according to an actuation sequence.The actuation sequence can be at least partially based on a relativeposition of each of the source elements within a particular geometry ofthe source subarray with respect to a previously actuated sourceelement, and a towing velocity of the source subarray.

The action sequence can be at least partially based on a time intervalbetween the actuations. The duration of the time interval can be lessthan a second. The duration of the time interval does not have to be thesame throughout the actuation sequence. For example, a first timeinterval between a first pair of consecutive actuations can have adifferent duration than a second time interval between a second pair ofconsecutive actuations.

The duration of the time interval can be predetermined. As used herein,“predetermined” is intended to mean that the duration of the timeinterval is a known value set prior to beginning the actuation sequence.For example, the duration of the time interval can be set to 0.1 secondsprior to beginning the actuation sequence.

The duration of the time interval can vary randomly. For example, theduration of a first time interval between a first pair of consecutiveactuations can be randomly different from the duration of a second timeinterval between a second pair of consecutive actuations.

The duration of the time interval can vary in a pseudo-random mannersuch that the durations of the time interval can vary randomly within amean time interval plus a randomization range and the mean time intervalminus the randomization range. If the mean time is 0.1 seconds and therandomization range is 0.05 seconds then the duration of the timeinterval can vary randomly between 0.95 seconds and 1.05 seconds. Forexample, the duration of a first time interval between a first pair ofconsecutive actuations can be 0.97 seconds and the duration of a secondtime interval between a second pair of consecutive actuations can be1.01 seconds.

FIG. 15 illustrates a system 1590 for individual actuation within asource subarray. The system 1590 can comprise a source subarray 1592comprising source elements 1551-1 through 1551-n. The source subarray1592 can be analogous to any source subarray in accordance with thepresent disclosure, including but not limited to those illustrated inFIGS. 8A-12B. The arrangement of the source elements 1551-1 through1551-n is not meant to limit the particular geometry of the sourcesubarray 1592. A controller 1591 can be coupled to the source subarray1592. The controller 1591 can be configured to actuate the sourceelements 1551-1 through 1551-n individually according to an actuationsequence. The actuation sequence can be at least partially based on arelative position of each source element 1551 within a particulargeometry of the source subarray 1592 with respect to a previouslyactuated source element 1551; and a towing velocity of the sourcesubarray 1592.

The source elements 1551-1 through 1551-n can be air guns. The sourcesubarray 1592 can comprise the air guns 1551-1 through 1551-n arrangedin a particular geometry. The controller 1591 can be configured toactuate the air guns 1551-1 through 1551-n individually according to anactuation sequence that is at least partially based on a towing velocityof the source subarray 1592 such that the actuation of each of the airguns 1551-1 through 1551-n occurs at least partially outside bubblesformed by previous actuations of the air guns 1551-1 through 1551-naccording to the actuation sequence; and refill each of the air guns1551-1 through 1551-n individually after each of the air guns 1551-1through 1551-n has been actuated such that the actuations of each of theair guns 1551-1 through 1551-n are continuous or near continuous.Refilling each of the air guns 1551-1 through 1551-n individually asopposed to refilling the air guns 1551-1 through 1551-n together canenable more efficient use of a compressor because the compressor canfully refill the air guns 1551-1 through 1551-n because the compressoris refilling one air gun instead of n air guns. Thus, the overall powerover time of the source subarray 1592 where the air guns 1551-1 through1551-n are refilled and actuated individually can be greater the powerof the source subarray 1592 where the air guns 1551-1 through 1551-n arerefilled and actuated simultaneously. The controller 1591 can be furtherconfigured to actuate the air guns 1551-1 through 1551-n with a timeinterval between the actuations. The time interval can be such that theactuation of each of the air guns 1551-1 through 1551-n occurs at leastpartially outside bubbles formed by a previously actuated air gun 1551-1through 1551-n according to the actuation sequence.

In accordance with a number of embodiments of the present disclosure, ageophysical data product may be produced. The geophysical data productmay include, for example, a marine seismic survey measurement with anestimated acquisition effect removed therefrom. Geophysical data may beobtained and stored on a non-transitory, tangible computer-readablemedium. The geophysical data product may be produced by processing thegeophysical data offshore or onshore either within the United States orin another country. If the geophysical data product is produced offshoreor in another country, it may be imported onshore to a facility in theUnited States. In some instances, once onshore in the United States,geophysical analysis may be performed on the geophysical data product.In some instances, geophysical analysis may be performed on thegeophysical data product offshore. For example, source elements of asource subarray can be individually actuated according to an actuationsequence. The actuation sequence can be at least partially based on arelative position of each of the source elements within a particulargeometry of the source subarray with respect to a previously actuatedsource element and a towing velocity of the source subarray.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Various advantages of the present disclosurehave been described herein, but embodiments may provide some, all, ornone of such advantages, or may provide other advantages.

In the foregoing Detailed Description, some features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thus,the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment.

What is claimed is:
 1. A method for individual actuation within a sourcesubarray, comprising: individually actuating source elements of a sourcesubarray according to an actuation sequence, wherein the actuationsequence is at least partially based on: a relative position of each ofthe source elements within a particular geometry of the source subarraywith respect to a previously actuated source element; and a towingvelocity of the source subarray.
 2. The method of claim 1, wherein theactuation sequence is at least partially based on a time intervalbetween the actuations, wherein a duration of the time interval is lessthan a second.
 3. The method of claim 1, wherein the actuation sequenceis at least partially based on a time interval between the actuations,wherein a duration of the time interval varies randomly.
 4. The methodof claim 1, wherein the actuation sequence is at least partially basedon a time interval between the actuations, wherein a duration of thetime interval varies randomly within a mean time interval plus arandomization range and the mean time interval minus the randomizationrange.
 5. The method of claim 1, wherein the actuation sequence is atleast partially based on a time interval between the actuations, whereinthe time interval has a predetermined duration.
 6. The method of claim1, wherein the actuation sequence comprises actuating the sourceelements with a first time interval between a first pair of consecutiveactuations and a second time interval between a second pair ofconsecutive actuations, wherein the first time interval has a differentduration than the second time interval.
 7. The method of claim 1,further comprising: repeating the actuation of the source elementsaccording to the actuation sequence after all of the source elements ofthe source subarray have been actuated.
 8. The method of claim 1,further comprising: actuating source elements of additional sourcesubarrays according to the actuation sequence, wherein the actuationsequence includes actuating a first source element in the sourcesubarray and each of the additional source subarrays before actuating asecond source element in each of the source subarrays.
 9. A system forindividual actuation within a source subarray, comprising: a sourcesubarray comprising source elements; and a controller coupled to thesource subarray and configured to actuate the source elementsindividually according to an actuation sequence, wherein the actuationsequence is at least partially based on: a relative position of each ofthe source elements within a particular geometry of the source subarraywith respect to a previously actuated source element; and a towingvelocity of the source subarray.
 10. The system of claim 9, wherein theparticular geometry comprises the source elements arranged in asubstantially elliptical shape.
 11. The system of claim 9, wherein theparticular geometry comprises the source elements arranged in asubstantially rectangular shape.
 12. The system of claim 9, wherein theparticular geometry comprises four of the source elements in a singleinline position along the source subarray.
 13. The system of claim 9,wherein the particular geometry comprises a first source element at afirst cross-line position and a second source element at a secondcross-line position, wherein the first cross-line position is differentthan the second cross-line position.
 14. The system of claim 9, whereinthe particular geometry comprises a first source element at a firstdepth and a second source element at a second depth, wherein the firstdepth is different than the second depth.
 15. The system of claim 9,wherein the particular geometry comprises the source elements arrangedalong at least one line.
 16. The system of claim 15, wherein the atleast one line is a single line; wherein an initial source element ofthe source elements is disposed at an inline position along the singleline that is closest to a water surface; and wherein the actuationsequence includes actuating the source elements beginning with theinitial source element followed by actuating, in order along the singleline, the source elements positionally subsequent to the initial sourceelement.
 17. The system of claim 15, wherein the at least one linecomprises a first line and a second line, wherein the first and secondlines share an endpoint; and wherein an angle between the first andsecond lines is less than 180 degrees.
 18. The system of claim 15,wherein the at least one line comprises a first line and a second line,wherein the first and second lines are substantially vertical withrespect to a water surface and substantially parallel to each other. 19.The system of claim 9, wherein the controller is further configured torepeat the actuation of the source elements according to the actuationsequence after all of the source elements of the source subarray havebeen actuated.
 20. The system of claim 9, further comprising: a sourcearray, wherein the source array comprises the source subarray and anadditional source subarray; and wherein the controller is configured toactuate source elements of the additional source subarray according tothe actuation sequence.